Not really sure that there's really any point to kids that young doing Valentine's Day things.
But "- A note from a mom sharing her contact info and an invitation for a playdate" This one is awesome! Our schools can't share parent contact info, so if you want to link up with any of your kids' classmates' parents, you have to go full stalker. Maybe park by the school, watch for the particular child to come out and get picked up, and then chase them down after school, or try to find/message them on FB, etc. Or give your child a note with your contact info and have them get it to the other kid... and hope it makes it to a parent so that you can actually connect. Our school also restricts bday invitations to the whole class, or nobody... so yet another obstacle to connecting with your kids' friends' parents.
I'd like to plan play dates for my kids with the kids they go to school with and like, but finding a way to actually connect with the parents isn't easy.